Radiation Physics and Chemistry 88 (2013) 65–69

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Radiation Physics and Chemistry

0969-80http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/radphyschem

Simultaneous radiation induced graft polymerizationof N-vinyl-2-pyrrolidone onto polypropylene non-woven fabric forimprovement of blood compatibility

Rong Li a,b, Hengdong Wang a, Wenfeng Wang a, Yin Ye a,n

a Shanghai Institute of Applied Physics, Chinese Academy of Sciences, No. 2019, Jialuo Road, Jiading District, Shanghai 201800, PR Chinab University of Chinese Academy of Sciences, No. 19, Yuquan Road, Shijingshan District, Beijing 100049, PR China

H I G H L I G H T S

� N-vinyl-2-pyrrolidone was grafted onto PPNWF via co-irradiation induced graft polymerization.

� The grafting of PVP enhanced the hydrophilicity of PPNWF and the DG can be simply controlled.� The modified PPNWF possessed good blood compatibility.

a r t i c l e i n f o

Article history:Received 17 February 2013Accepted 1 March 2013Available online 15 March 2013

Keywords:Radiation induced graft polymerizationPolypropylene non-woven fabricN-vinyl-2-pyrrolidoneBlood compatibility

6X/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.radphyschem.2013.03.013

esponding author. Tel.: þ86 21 39194130; faxail address: yeyin@sinap.ac.cn (Y. Ye).

a b s t r a c t

In this study, N-vinyl-2-pyrrolidone (NVP) was grafted onto polypropylene non-woven fabric (PPNWF)through a simultaneous irradiation induced graft polymerization technique. Effect of the parameters ofgraft polymerization, i.e., monomer concentration, absorbed dose and dose rate, on the degree of grafting(DG) was investigated. The graft polymerization of NVP was confirmed by attenuated total reflectanceFourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS).A contact angle goniometry was used to test water contact angle (WCA) of original PPNWF and modifiedsamples. The in vitro blood compatibility, including hemolysis, protein adsorption, platelet adhesion andactivated partial thromboplastin time (APTT) of tested specimens, was evaluated. The results demon-strated that the hemocompatibility of PPNWF was improved via graft polymerization of NVP.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Modification via γ-ray radiation induced graft polymerization isa good choice for those materials without reactive groups, and thismethod offers an easy and highly efficient way to modify itsproperties (Cai et al., 2012) due to rapid formation of active siteson the substrate surface and in the material matrix during thegrafting process (Plessier et al., 1998; El-Sawy and Elassar, 1998).

Polypropylene non-woven fabric (PPNWF) has some excellentproperties as used in the field of blood purification, such as non-toxicity, chemical stability and low cost (Wang et al., 2010; Jin et al.,2012). However, the poor hydrophilicity of unmodified PPNWF, whichcan induce blood protein adsorption, platelet adhesion, etc., limited itsfurther application in blood separation field (Song et al., 2011;Zhao et al., 2010). Therefore, it is of much importance to improve

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hydrophilicity of polypropylene nonwoven fabric for improvement ofblood compatibility.

N-vinyl-2-pyrrolidone, as a hydrophilic and non-ionic monomer,had been widely used to modify different materials via various kindsof grafting methods, such as 60Co γ-ray pre-irradiation (Liu et al.,2004; Xu et al., 2012), low temperature plasma treatment (Chen andBelfort, 1999), UV photo irradiation (Liu et al., 2004; Luan et al., 2012)and surface-initiated atom transfer radical polymerization (ATRP)(Feng et al., 2011). The wide research and application of NVP could beascribed to its good hydrophilicity and excellent biocompatibility(Wan et al., 2005; Wetzels and Koole, 1999).

In this work, we grafted NVP onto PPNWF via 60Coγ-ray co-irradiation induced graft polymerization. The graft poly-merization of NVP was checked via of ATR-FTIR and XPS. Thehydrophilicity was examined by a contact angle goniometry. Theblood compatibility, including hemolysis, protein adsorption, plateletadhesion and activated partial thromboplastin time (APTT), wasevaluated.

R. Li et al. / Radiation Physics and Chemistry 88 (2013) 65–6966

2. Experimental

2.1. Materials

PPNWF (130 g/m2) was kindly offered by Shanghai ShilongHi-tech Co., Ltd. NVP was purchased from Sigma-Aldrich ChemicalCo. Bovine serum albumin (BSA) was bought from SinopharmChemical Reagent Co. Ltd., Shanghai, China. Human whole blood,platelet rich plasma (PRP) and platelet poor plasma (PPP) werekindly offered by Chengdu Blood Center. All other reagents wereanalytical grade and used without further purification.

2.2. Simultaneous radiation induced graft polymerization

PPNWF (10 cm�10 cm) was put into glass tubes, then deio-nized aqueous solution with NVP monomer was added, and thetubes were purged with nitrogen bubbling for 10 min. The glasstubes were sealed and then irradiated by γ-rays from a 60Cosource. The grafted PPNWFs were removed and washed with lotsof deionized water to remove residual homopolymer. Finally, thePPNWFs were dried in air dry oven until a constant weight wasobtained. The DGs were determined by the weight increaseaccording to the following equation:

DGð%Þ ¼ W1−W0

W0� 100 ð1Þ

where W0 and W1 are the weights of pristine and modifiedPPNWF, respectively.

2.3. Characterization analysis

ATR–FTIR spectra were evaluated on a Nicolet Avatar 380 FT-IRspectrometer by averaging 8 scans, at a resolution of 4 cm−1.

XPS analysis was performed with a Kratos Axis Ultra instru-ment using monochromatic Al Kα radiation. Scans in wide range of1100–0 eV were applied. The tested samples were vacuum-driedand then the XPS spectra were taken. Surface elemental composi-tions relative to carbon-1 s were calculated from the peak heightwith a correction for atomic sensitivity.

2.4. Static water contact angle measurement

The contact angles were measured on a THETA Optical Tensi-ometer (KSV Instruments Ltd., Finland) equipped with videocapture. A 5 μL water drop from a needle tip was placed onto thesample's surface. Every sample was measured at 5 different points,and the contact angle was the average of these data.

2.5. In vitro evaluation of blood compatibility

2.5.1. Hemolysis assayHemolysis assay was performed according to the procedure

systematically illustrated in the literature (Yang et al., 2011; Caoet al., 2011). 1.0 g PPNWF was cut into pieces before transferredinto polystyrene tube. 20 mL of 0.9% normal saline was pouredinto each of the tubes and kept at 37 ◦C in a shaking water bath.Then the red blood cell was washed three times by 0.9% normalsaline. After 4 h in incubation, 400 μL of washed red blood cell(2 mL red blood cell was diluted with 10 mL normal saline) wasadded into each of the tubes. All of the tubes were furtherincubated at 37 1C for 60 min. Similarly, positive and negativecontrols were produced in separate tubes by adding 400 μL of thediluted blood to 20 mL of distilled water and normal saline,respectively. The fluid was then transferred into a fresh polystyr-ene tube and centrifuged at 2500 rpm for 5 min. The absorbance ofsupernatants was measured at 545 nm using a spectrophotometer.

The hemolysis ratio was calculated according to the followingformula:

HRð%Þ ¼ As−Anc

Apc−Anc� 100 ð2Þ

where As, Apc and Anc are the absorbance of sample supernatant,positive control and negative control, respectively.

2.5.2. Protein adsorptionBovine serum albumin was used as a model protein to evaluate

the protein resistance of the samples in phosphate buffered saline(PBS, pH 7.4). The specimens (1 cm�1 cm) were placed in indivi-dual wells of a 24 well tissue culture plate. After equilibrated byPBS at 37 1C for 12 h, the specimens were immersed into two kindsof BSA solutions (1.0 and 2.0 mg/mL) at 37 1C for 2 h, respectively.The amount of adsorbed protein was determined by the Bradfordmethod as specifically described in the literature (Jin et al., 2012).

2.5.3. Platelet adhesionEach sample (1 cm�1 cm) was washed three times with

deionized water and placed into the wells of 24-well cultureplates in contact with 1 ml of human PRP containing500�109 cells/L of platelets and was incubated at 37 1C for60 min. The number of platelets adhered to the samples wasdetermined by the lactate dehydrogenase (LDH) method (Tamadaet al., 1995). A linear relationship was obtained between the LDHactivity of aliquots and the known concentration of plateletscounted by a Medonic Cell Analyser (CA 620). The adheredplatelets on the membranes were thus calculated with thecalibration curve.

2.5.4. APTTA sample film (1 cm�1 cm) was incubated in 1 mL of PPP at

37 1C for 1 h. The APTT of the PPP was determined using anautomated blood coagulation analyzer (CA-1500, Sysmex Corp.,Kobe, Japan) (Lin et al., 2004).

3. Results and discussion

3.1. Graft polymerization of N-vinyl-2-pyrrolidone

Fig.1apresented the relationshipbetween theDGand themonomerconcentration. As shown in Fig. 1a, the DG, as was expected, increasedwith increasing of monomer concentration. Besides, there is almost alinear relationship between the DG andmonomer concentration.

Fig. 1b displaced the relationship between the DG and the totalabsorbed dose. The DG increased with the growing absorbed dose,and the DG tended to be saturated by further escalating ofabsorbed dose more than 13 kGy. The result could be attributedto that the monomer in the solution had been depleted owing tocompetition between graft polymerization and homopolymeriza-tion of NVP in co-irradiated grafting system.

Fig. 1c showed the effect of dose rate on the DG of NVP ontoPPNWF. The peak DG was obtained when the dose rate was set at0.22 kGy/h. However, the DG did not present distinct differencewith the varying of dose rate from 0.11 kGy/h to 1.67 kGy/h. Wecould make a conclusion that the dose rate had little effect on DGof NVP in this co-irradiation grafting system.

3.2. Characterization of modified PPNWF

Fig. 2 depicted the ATR–FTIR spectra of neat PPNWF andmodified PPNWFs. In the IR spectra of PP-g-PVP (Fig. 2b and c),two characteristic peaks at about 1655 cm−1 and 1289 cm−1 couldbe assigned to the CQO stretching vibration of amide group and

0

50

100

150

200

Absorbed Dose = 10 kGyIrradiation time = 17 h

Deg

ree

of g

rafti

ng (%

)

CNVP (vol.%)

R2 = 0.986

0

15

30

45

Deg

ree

of g

rafti

ng (%

)

Absorbed dose (kGy)

Dose rate = 0.59 kGy/h

CNVP(vol.%) = 5%

0 5 10 15 20 0 8 16 24 32 40 48 56

0.0 0.3 0.6 0.9 1.2 1.5 1.80

10

20

30

40

50

Deg

ree

of g

rafti

ng (%

)

Dose rate (kGy/h)

Absorbed dose = 10 kGyCNVP (vol.%) = 5%

Fig. 1. Kinetic study result of the relationship between the DG and trial parameters of (a) concentration of NVP, (b) absorbed dose and (c) dose rate.

3500 3000 2500 2000 1500 1000

Wavenumber (cm-1)

1655

1289

(a)

(b)

(c)

Fig. 2. ATR–FTIR spectra of neat PPNWF (a) and PP-g-PVP samples with DG((b) DG¼8.22%; (c) DG¼17.38%).

1000 800 600 400 200 0

C1s

N1s(b) PP-g-PVP (DG = 16.52%)

Binding energy (eV)

(a) Nascent PPNWF

O1s

Fig. 3. XPS survey spectra of nascent PPNWF (a), and (b) PVP-modified PPNWF(DG¼16.52%).

R. Li et al. / Radiation Physics and Chemistry 88 (2013) 65–69 67

stretching vibration of C–N bond in the pyrrolidone ring, respec-tively (Xu et al., 2012; Liu et al., 2005). Furthermore, the intensityof these peaks became stronger with the increasing of DG asclearly portrayed in Fig. 2b and c. On the other hand, no such peakswere observed in pristine PPNWF (Fig. 2a). The above IR resultsconfirmed that the presence of PVP chains on PPNWF.

The XPS wide-scan spectra of nascent PPNWF and PP-g-PVPsample were exposed in Fig. 3, and the chemical compositions ofthe specimens were listed in Table 1. C and O elements werepresented in pristine PPNWF and the infinitesimal amount of Oelement in virgin PPNWF might be owing to the remainingadditive, which was not informed by the supplier. After graftingpolymerization, the N concentration evidently increased from 0%

to 10.70% and O content accordingly augmented from 1.69% to11.62%. These data also indicated the existence of PVP chainson PPNWF.

Static water contact angles of the modified PPNWFs werepresented in Fig. 4. The contact angle of pristine PPNWF was143.51. PVP graft chains are hydrophilic ascribed to the existenceof amide carbonyl functional groups, and therefore the modifiedPPNWFs are relatively hydrophilic. It is evident that the contactangles of modified PPNWFs went down with the escalating of DG.When the DG was 17.38%, the contact angle of the modifiedPPNWF was 34.41, which means that the hydrophilicity of thePVP-tethered PPNWFs was greatly improved.

Table 1Chemical composition of nascent and modified PPNWF by XPS analysis.

Sample Atomic percent (%)

C N O

Nascent PPNWF 98.31 – 1.69PP-g-PVP 77.68 10.70 11.62(DG¼16.52%)

0

20

40

60

80

100

120

140

160

D G = 17.38%D G = 8.22%

D G= 2.02 %D G = 0

Wat

er c

onta

ct a

ngle

(o )

Fig. 4. Static water contact angles of the modified PPNWFs. Values are expressed asmeans 7 SD, n¼5.

-5

-4

-3

-2

-1

0

1

2

3

4

5D

G = 87.85%

DG

= 65.59%

DG

= 53.20%

DG

= 43.44%

DG

= 28.27%

DG

= 15.02%

DG

= 6.61%

Hem

olys

is ra

te (%

)

DG

= 0Fig. 5. Hemolysis rates of the modified samples. Values are expressed asmeans 7 SD, n¼3.

200

300

400

500

2mg/mL

BSA

ads

orpt

ion

(10-3

mg/

cm2 )

Original PPNWF DG = 7.92% DG = 16.52% DG = 29.44%

1mg/mL

BSA concentration

Fig. 6. BSA adsorption on the original PPNWF and the PVP modified PPNWF. Valuesare expressed as means 7 SD, n¼3.

3

6

9

12

15

DG = 65.59%

DG = 41.62%

DG = 29.44%

DG = 16.52%

DG = 7.92%DG = 0

Plat

elet

adh

esio

n (1

05 pla

tele

ts /c

m2 )

Fig. 7. Amount of platelet adhered on the modified PPNWFs. Values are expressedas means 7 SD, n¼3 (Initial concentration of platelets was 500�109 platelets/L.).

R. Li et al. / Radiation Physics and Chemistry 88 (2013) 65–6968

3.3. In vitro evaluation of blood compatibility

3.3.1. Hemolysis assayThe hemolysis assay was adopted to evaluate the materials'

erythrocyte compatibility. The hemolysis rate (HR) of PP-g-PVPsamples with different DG was given in Fig. 5. The result indicatedthat modified PPNWFs possessed better erythrocyte compatibilitythan original PPNWF. Nevertheless, a PP-g-PVP sample with DG15.02% showed the unexpected result as compared to that of theother modified samples, which might be attributed to the experi-mental error. This consequence manifested that immobilization ofPVP onto PPNWF could improve blood compatibility.

3.3.2. Protein adsorptionProtein adsorption is a complicated process, though it is one of

the important factors in estimating the hemocompatibility ofmaterials (Ishihara et al., 1999). Materials possessing hydrophili-city generally showed relatively low non-selective adsorption forproteins (Nie et al., 2012). Therefore, in this work, the BSAadsorption was performed to evaluate blood compatibility oftested samples. It can be seen in Fig. 6 that BSA proteins were

absorbed on the original PPNWF (347.5 and 488.8 μg/cm2) in both1.0 and 2.0 mg/mL BSA solutions owning to its hydrophobicity.After graft polymerization, the adsorption of BSA on the modifiedspecimens can be significantly reduced with the growing of DGwith both BSA concentrations. Besides, the restricted efficiency ofBSA adsorption was much more obviously exhibited in 2 mg/mLsolution as compared to that in 1 mg/mL solution. However, theBSA adsorption on the modified samples did not illustrate distinctdecline with increasing of DG from 16.52% to 29.44%. These resultsconfirmed that modified PPNWF with DG of 16.52% was sufficientto restrain the protein adsorption. We could make a conclusionthat the restraint of proteins adsorption might be ascribed to theimmobilization of hydrophilic PVP-chains.

3.3.3. Platelet adhesionPlatelet adhesion plays an important role in the aspect of

evaluation of materials' blood compatibility. This work alsoemployed this test to evaluate the blood compatibility of modifiedPPNWFs. Fig. 7 illustrated the number of platelets adhered on thePP-g-PVP samples with different DG. As displaced in Fig. 7, a lot ofplatelets (11�105 /cm2) adhered on unmodified PPNWF. Never-theless, the amount of aggregated platelets was suppressed from9.2�105 /cm2 (DG, 7.92%) to 4.2�105 /cm2 (DG, 65.59%) afteranchored with PVP chains. This result indicated that the hemo-compatibility of PPNWF was efficiently improved via immobiliza-tion of PVP, which could be ascribed to the hydrophilicity andbiocompatibility of PVP. When contacting with the platelets, thePVP chains became hydrated, which had exerted steric hindranceeffects to the approaching of platelets (Liu et al., 2005).

30

32

34

36

38

40

negative control

DG = 87.85%

DG = 65.59%

DG = 41.62%

DG = 16.52%DG = 7.92%

DG = 0

APT

T (s

ec)

Fig. 8. The APTT of grafted PPNWFs. Values are expressed as means7SD, n¼3.

R. Li et al. / Radiation Physics and Chemistry 88 (2013) 65–69 69

3.3.4. APTTThe blood coagulation cascade included intrinsic pathway,

extrinsic pathway, and common pathway (Lin et al., 2004). APTTis used to examine mainly the intrinsic pathway and detectsfunctional deficiencies in factor II, V, X, or fibrinogen (Kamalet al., 2007). A kind of material contacted with blood might incurthe elongation of APTT due to combination or reaction withcoagulation factors of blood (Nie et al., 2012). Materials presentinggood blood compatibility often showed litter effect on elongationof APTT (Cao et al., 2011).

In this work, the APTT was applied to evaluate the antic-oagulation activity of neat PPNWF and grafted samples. Fig. 8described that the APTT of nascent PPNWF (31.4 s) was nearly thesame as that of human plasma (the negative control, 31.3 s),whereas the APTT of PP-g-PVP samples was slightly lengthenedfrom 32.4 s to 36.4 s. This result demonstrated that the PVP-modified PPNWF samples had lower effect on coagulation factorsand presented good blood compatibility.

4. Conclusion

In order to improve the blood compatibility of PPNWF, N-vinyl-2-pyrrolidone, a hydrophilic monomer, was successfully graftedonto PPNWF via co-irradiation induced graft polymerization,which was confirmed by ATR–FTIR and XPS. Graft polymerizationkinetics was studied and the degree of grafting was adjusted viaaltering the monomer concentration or absorbed dose. The hydro-philicity was greatly enhanced after the graft polymerizationaccording to the decrease of WCA. The modified PPNWFs pre-sented good blood compatibility (lower hemolysis rate, lowerprotein adsorption, lower platelet adhesion and lower effect oncoagulation factors) as compared with the pristine PPNWF, due tograft of hydrophilic (poly-N-vinyl-2-pyrrolidone) chain. Theseresults indicated that the PVP-modified PPNWFs might be apromising material used in the field of blood purification.

Acknowledgments

The authors are very appreciative of technical assistance aboutevaluation of materials' blood compatibility from Institute of BloodTransfusion, Chinese Academy of Medical Sciences and PekingUnion Medical College.

References

Cai, R., Deng, B., Jiang, H.Q., Yu, Y., Yu, M., Li, L.F., Li, J.Y., 2012. Radiation inducedgraft polymerization of a fluorinated acrylate onto fabric. Radiat. Phys. Chem.81, 1354–1356.

Cao, Y., Wang, H., Yang, C., Zhong, R., Lei, Y., Sun, K., Liu, J.X., 2011. In vitro studies ofPBT nonwoven fabrics adsorbent for the removal of low density lipoproteinfrom hyperlipemia plasma. Appl. Surf. Sci. 257, 7521–7528.

Chen, H., Belfort, G., 1999. Surface modification of poly(ether sulfone) ultrafiltrationmembranes by low-temperature plasma-induced graft polymerization. J. Appl.Polym. Sci. 72, 1699–1711.

El-Sawy, N.M., Elassar, A.Z.A., 1998. Some modification on radiation graft poly-merization of N-vinyl-2-pyrrolidone onto low density polyethylene withα,β-unsaturated nitrile. Eur. Polym. J. 34, 1073–1080.

Feng, R.R., Wang, C.C., Xu, X.C., Yang, F.L., Xu, G.J., Jiang, T., 2011. Highly effectiveantifouling performance of N-vinyl-2-pyrrolidone modified polypropylenenon-woven fabric membranes by ATRP method. J. Membr. Sci. 369, 233–242.

Ishihara, K., Fukumoto, K., Iwasaki, Y., Nakabayashi, N., 1999. Modification ofpolysulfone with phospholipid polymer for improvement of the blood compat-ibility. Part 2 Protein adsorption and platelet adhesion. Biomaterials 20,1553–1559.

Jin, J., Jiang, W., shi, Q., Zhao, J., Yin, J.H., Stagnaro, P., 2012. Fabrication of PP-g-PEGMA-g-heparin and its hemocompatibility: From protein adsorption toanticoagulant tendency. Appl. Surf. Sci. 258, 5841–5849.

Kamal, A.H., Tefferi, A., Pruthi, R.K., 2007. How to interpret and pursue an abnormalprothrombin time, acticated partial thromboplastin time, and bleeding time inadults. Mayo Clin. Proc. 7, 864–873.

Lin, W.C., Liu, T.Y., Yang, M.C., 2004. Hemocompatibility of polyacrylonitrile dialysismembrane immobilized with chitosan and heparin conjugate. Biomaterials 25,1947–1957.

Liu, Z.M., Xu, Z.K., Wang, J.Q., Wu, J., Fu, J.J., 2004. Surface modification ofpolypropylene microfiltration membranes by graft polymerization of N-vinyl-2-pyrrolidone. Eur. Polym. J. 40, 2077–2087.

Liu, Z.M., Xu, Z.K., Wan, L.S., Wu, J., Ulbricht, M., 2005. Surface modification ofpolypropylene microfiltration membranes by the immobilization of poly(N-vinyl-2-pyrrolidone): a facile plasma approach. J. Membr. Sci. 249, 21–31.

Luan, S.F., Zhao, J., Yang, H.W., Shi, H.C., Jin, J., Li, X.M., Liu, J.C., Wang, J.W., Yin, J.H.,Stagnaro, P., 2012. Surface modification of poly(styrene-b-(ethylene-co-buty-lene)-b-styrene) elastomer via UV-induced graft polymerization of N-vinyl-2-pyrrolidone. Colloids Surf. B 93, 127–134.

Nie, S.Q., Xue, J.M., Lu, Y., Liu, Y.Q., Wang, D.S., Sun, S.D., Ran, F., Zhao, C.S., 2012.Improved blood compatibility of polyethersulfone membrane with a hydro-philic and anionic surface. Colloids Surf. B 100, 116–125.

Plessier, C., Gupta, B., Chapiro, A., 1998. Modification of polypropylene fiber byradiation-induced graft copolymerization of acrylonitrile monomer. J. Appl.Polym. Sci. 69, 1343–1348.

Song, L.J., Zhao, J., Yang, H.W., Jin, J., Li, X.M., Stagnaro, P., Yin, J.H., 2011.Biocompatibility of polypropylene non-woven fabric membrane via UV-induced graft polymerization of 2-acrylamido-2-methylpropane sulfonic acid.Appl. Surf. Sci. 258, 425–430.

Tamada, Y., Kulik, E.A., Ikada, Y., 1995. Simple method for platelet counting.Biomaterials 16, 259–261.

Wan, L.S., Xu, Z.K., Huang, X.J., Wang, Z.G., Wang, J.L., 2005. Copolymerization ofacrylonitrile with N-vinyl-2-pyrrolidone to improve the hemocompatibility ofpolyacrylonitrile. Polymer 46, 7715–7723.

Wang, C.C., Yang, F.L., Zhang, H.M., 2010. Fabrication of non-woven compositemembrane by chitosan coating for resisting the adsorption of proteins and theadhesion of bacteria. Sep. Purif. Technol. 75, 358–365.

Wetzels, G.M.R., Koole, L.H., 1999. Photoimmobilisation of poly (N-vinylpyrrolidi-none) as a means to improve haemocompatibility of polyurethane biomaterials.Biomaterials 20, 1879–1887.

Xu, C.Q., Huang, W., Zhou, Y.F., Yan, D.Y., Chen, S.T., Huang, H., 2012. Graftcopolymerization of N-vinyl-2-pyrrolidone onto pre-irradiated poly(vinylidenefluoride) powder. Radiat. Phys. Chem. 81, 426–431.

Yang, C., Cao, Y., Sun, K., Liu, J.X., Wang, H., 2011. Functional groups graftednonwoven fabrics for blood filtration—the effects of functional groups andwettability on the adhesion of leukocyte and platelet. Appl. Surf. Sci. 257,2978–2983.

Zhao, J., Shi, Q., Yin, L.G., Luan, S.F., Shi, H.C., Song, L.J., Yin, J.H., Stagnaro, P., 2010.Polypropylene modified with 2-hydroxyethyl acrylate-g-2-methacryloyloxyethylphosphorycholine and its hemocompatibility. Appl. Surf. Sci. 256, 7071–7076.